Linear motor compressors and compressor drives are known in the prior art. U.S. Pat. Nos. 3,891,874, 3,937,600, 4,067,667, and 4,345,442 describe linear compressors having provisions for piston stroke control. Applications of such linear compressors include their use in heat pumps, as described in U.S. Pat. No. 4,345,442. These heat pumps are used for space heating and cooling, as in applications in dwellings. U.S. Pat. No. 4,345,442 describes a linear motor drive for the compressor wherein a selectively variable mechanical resonant frequency of the pump is said to be generally centered in the power line frequency of the alternating current power source which is used to drive the linear motor. These patents describe an electrodynamic type of linear oscillating motor for the piston drives in the compressors. These linear motor drives have both AC and DC field coil windings. Usually, a fixed direct current is coupled to the DC field windings over the complete operating range of the motors. An exception to this is described in U.S. Pat. No. 4,291,258 in which a control system is provided which couples a DC coil current to the DC field coils, which is said to be quasi-proportional to the magnitude of the AC field coil current.
While DC is used as field coil current in the systems of these patents, it is not the sole power source. It is used only to provide a DC field with which the AC field interacts. The systems described in these patents are not miniaturized systems and are not adapted for miniaturization, nor are they adapted for use with DC power supplies as the sole power source. Where a DC power supply is used, different techniques for energizing and controlling the linear motor are required. These are neither described nor suggested in the referenced patents.